OpenServoCore - Migration from STM32 to CH32 RISC-V

Switching from STM32F301 to CH32V303 to cut BOM cost.

Pros:

• $0.80 vs $2.40 per unit
• RISC-V core (learning opportunity)
• Same peripherals (ADC, timers, UART)

Cons:

• Toolchain less mature
• Need to port HAL code
• Debugging might be painful

Started porting. ADC and PWM working. Timer interrupts next.